What is a PLC

1.  The PLC was designed to work for years in the more extreme conditions you might find in a factory.

2.  The PLC was designed to sense a large number of inputs and control a large number of outputs all in a matter of milliseconds.

3.  The PLC uses a symbolic programming language that makes it easier to understand and change how all those inputs will influence all those outputs.
 

Germany first used ladder Logic in the 1930's to show in a symbolic way how an electrical sequence would work.  This type of schematic looks like the rails and rungs of a ladder.  The rails are symbolic of the electrical power source.  On the left side of a rung would be the inputs - normally open and closed contacts symbolic of the limit switches, timer contacts, push buttons, etc.  On the right side of a rung would be the output coil symbolic of the electric solenoid, motor, electric valve, etc. that would be influenced by the inputs.  Power from the left rail could flow through the input contacts to the output coil connected to the right rail.

Ladder Logic helped in understanding the "sequence of operation" but thousands of hours would be required to make changes in the sequence to produce next year's car model.  The relay, timer, and counter contacts had to be disconnected and reconnected in a different way for every model change!

The PLC concept of manufacturing put a stop to all this rewiring from one model year to the next.  The assembly line was made flexible by routing all inputs to the PLC and all outputs from the PLC.  To change the sequence, you change the Ladder Logic program that's running in the PLC! - A Software Change Instead of a Hardwire Change!  The PLC also did away with all the electro-mechanical timers and counters that were required by using the computer clock and math functions contained within the PLC.  This reduced down time on the assembly line by a factor of 10 because most of the mechanical devices previously used in affecting the sequence were replaced by a Software Program!
 

Remember the 90-90 rule:  90% of the PLCs can handle 90% of the applications




If you were given the task to specify a PLC for an application, what are two key questions you need to answer first?  

1.  How much I/O is needed for the application?  Each PLC has a limit on how many inputs and outputs it can control.

2.  How much program memory is needed to run the PLC for that application?  Most PLCs have a fixed amount of memory where you store the ladder logic program.  Some PLCs have interchangeable memory.


There are three types of PLC configurations.

 
1.  Modular systems have plug in input and output modules.  This makes it easy to add more I/O or change units.  You can choose AC or DC and special modules (analog, motor control or ASCII port).

2.  Link distributed I/O systems allow hundreds of feet of separation between each I/O station using coax, twisted pair of fiber optic connections between stations.  This saves a lot of labor and wire costs.

3.  Fixed base PLCs have a fixed amount of I/O built into the same housing as the CPU and power supply.  Some fixed base (and modular) can be linked also.  You can specify different methods of output:

 
a.  Relay contact
b.  Transistor (solid state DC)
c.  Triac (solid state AC)

There are three methods of programming a PLC.   1.  Single instruction programmer.  This programmer is useful for a small edit of an existing program.

2.  Dedicated manufactures programming unit.  These units have a screen to show a page of ladder logic at one time.  You can use these units to monitor the PLC while it is running in addition to edit of the program.

3.  IBM compatible software to allow a desktop or laptop computer to be the programming device.  Most of the features of the dedicated units plus good documentation features make this the more popular method of programming.
  PLCs today have very powerful instruction sets far beyond the original contact and coil logic.  Less than 1% of the population have ever read or heard of the phrase: Programmable Logic Controller,  but the past, present, and future impact of the PLC is beyond calculation.